libxenctrl headers should not pollute macro namespace with

libxenctrl headers should not pollute macro namespace with
mb/rmb/wmb. Instead add a xen_ prefix. Modify Xen's public headers to
expect the prefixed names instead of bare mb/rmb/wmb, but gate this
expectation on a bump of __XEN_INTERFACE_VERSION__.
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>